1. Field of the Invention
The present invention relates to an ultrasonic transducer that transmits or receives ultrasonic waves.
2. Description of the Related Art
Ultrasonic transducers are used as back sonar of automobiles. Ultrasonic transducers according to the related art include a case having a bottomed, substantially cylindrical shape that is closed at an end in the main axis direction, a piezoelectric element bonded to the inner bottom surface of the case, resin that blocks the opening of the case, and the like. Ultrasonic transducers apply a driving voltage to the piezoelectric element to cause the piezoelectric element and the case to vibrate to thereby transmit ultrasonic waves toward the outside of the case, receive reflected waves bounced back from a target, and measure the reflection time, thereby measuring the distance to the target.
In such ultrasonic transducers, ultrasonic waves are transmitted not only toward the outside of the case but also toward the inside of the case. The ultrasonic waves transmitted toward the inside of the case bounce back toward the piezoelectric element upon reaching the resin, causing the piezoelectric element to vibrate again. These excess vibrations are recognized as reverberation. Generally, in such a case, the reverberation time of the ultrasonic transducers tends to become long since the ultrasonic waves undergo multiple reflections several dozen times between the resin and the piezoelectric element. Longer reverberation time makes short-distance detection more difficult.
An ultrasonic transducer that can solve such a problem is disclosed in International Publication No. 2007/029559, for example. As shown in FIG. 12, an ultrasonic transducer 700 disclosed in International Publication No. 2007/029559 includes a case body 71, a piezoelectric element 72, a base substrate 73, lead wires 74, external connection terminals 75, and a sound-absorbing material 70.
The case body 71 has a bottomed, substantially cylindrical shape that is closed at an end in the main axis direction, and is formed from metal. The case body 71 includes an outer case 76 having a bottomed, substantially cylindrical shape, and an inner case 77 having a substantially cylindrical shape provided on the inner periphery of the outer case 76. The piezoelectric element 72 is bonded to the inner bottom surface of the case body 71.
The sound-absorbing material 70 is opposed to the piezoelectric element 72, and is placed in the space inside the case body 71 at a spacing from the piezoelectric element 72 so that the sound-absorbing material 70 does not come into contact with the main surface of the piezoelectric element 72. The sound-absorbing material 70 is formed from porous silicone.
The base substrate 73 is provided on the other main surface of the sound-absorbing material 70. Two lead wires 74 are connected to the base substrate 73, one to one electrode of the piezoelectric element 72, the other to the case body 71. Also, two external connection terminals 75 connected to the lead wires 74 are connected to the base substrate 73. The external connection terminals 75 are led out to the outside of the case body 71.
The related art illustrated in FIG. 12 achieves an improvement in reverberation characteristic by provision of the sound-absorbing material in the interior of the case. However, even such a measure cannot completely eliminate reverberation of ultrasonic waves. In some cases, a further improvement in reverberation characteristic is desired.